POE is a revolutionary technology that integrates data, voice and power over standard Ethernet infrastructure. POE is a means for supplying reliable, uninterrupted power to Internet Protocol (IP) telephones, wireless LAN access points, network cameras and other Ethernet devices, using existing, commonly used network cable infrastructure. POE technology saves time and expense related to the installation of separate power cabling, AC outlets and ‘wall warts’, which is a term used in the industry referring to a transformer, or power supply, that plugs into a wall outlet for converting the AC power from the outlet into DC power for an electronic device, such as a modem or a charging station for a laptop computer, etc. POE also eliminates the need for a dedicated Uninterruptible Power Supply (UPS) for individual devices.
The power delivered over the Ethernet infrastructure is automatically activated when a compatible terminal is identified, and blocked to devices that are not POE compatible. This feature allows users to freely and safely mix both POE-compatible and non-POE-compatible devices on their network.
POE devices typically comply with the IEEE802.3af standard. The IEEE802.3af standard is presently a draft standard that will ultimately be incorporated within IEEE 802.3. The technology driving IEEE802.3af is designed in a way that does not degrade the network data communication performance or decrease the network reach. POE devices include, among other things, Power Sourcing Equipment (PSE) and Powered Devices (PDs).
There are two main implementations of POE: (1) The end-span PSE—A POE enabled Ethernet switch by which power is supplied directly from the data ports; and (2) The mid-span PSE—A patch-panel like device, residing between an ordinary Ethernet switch and the terminals, often referred to as a “Power Hub”, by which power is added onto the spare wires, with data uninterrupted. A PD in compliance with IEEE 802.3af receives its power from either the end-span device, i.e., over the data lines, or from a mid-span device, i.e., over the spare wires.
A portion of a PSE includes a mixed signal control section 10 and a power section 20, as shown in FIG. 1. The power section 20 typically includes a power switch 21, a protective device 22 and a current sensing device 23. The power switch can be implemented by a MOSFET, a bipolar transistor, a relay, etc. The protective device can be a fuse, an electro-mechanical circuit breaker or a positive temperature coefficient (PTC) device. Lastly, the current sensing device is typically implemented by a low ohmic-value resistor.
The power switch 21 controls the amount of current supplied to the load, or PD, based on information from the current sensing device 23 or, in the case of a current sensing resistor, by measuring voltage across this resistor. The power switch 21 can also perform a protective function, for example, by first limiting the current drawn by the load and then, ultimately, disconnecting the load under abnormal conditions, such as an overload or short circuit. Additionally, in conventional POE equipment, a protective device, such as a fuse, protects the internal circuitry and wiring of the POE equipment from an overload or short circuit condition in the event of a short circuit catastrophic failure of the power switch in conjunction with an overload condition in either or both of the PD circuitry and the cabling 30 connected to the PD.
UL 60950 is a safety standard written by Underwriters Laboratories (UL), a private company that drafts standards for use in certifying product safety in the U.S. and also independently tests products to verify compliance with those standards. UL 60950 is entitled “Safety of Information Technology Equipment, Including Electrical Business Equipment” and is harmonized to the international standard IEC 60950 with the same title. A European version of the UL 60950 standard also exists as EN60950, as does other international standards corresponding to various countries. All of these standards, and at least all of the related 60950 standards, are hereby incorporated by reference for all that they teach. Additionally, although the UL 60950 standard is specifically referenced throughout the remainder of this disclosure, all other related 60950 standards are equally applicable.
According to UL 60950 and the general POE standard published by the IEEE (IEEE802.3af), which is also incorporated hereby by reference for all that it teaches, POE devices must comply with the safety requirements of so-called limited power sources (LPS). These safety industry standards specify the requirements for the maximum allowable electrical current and power output from the LPS device. Specifically, the standards specify the maximum allowable output current and power measured sixty (60) seconds after the application of any non-capacitive load, including a short circuit. In regard to POE devices with an output voltage of 50V, the maximum output current with any non-capacitive load applied, measured sixty (60) seconds after application of the load, must not exceed two (2) amperes (A) current or 100 volt-amps (VA) power.
In addition to the several safety standards mentioned above, the U.S. Department of Labor's Occupational Safety and Health Administration (OSHA) regulations require that all electrical equipment used in industrial settings be approved by product safety standards. Moreover, “approved” means that the equipment has been tested and verified to generally accepted UL or American National Standards Institute (ANSI) testing standards. Thus, adherence to UL 60950 and similar standards is very important, as is providing a cost-effective design for POE equipment.